Many applications require a steel part having a hardened outer surface and an interior region of lower hardness to provide improved strength, wear resistance, and toughness. Induction case hardening is a commonly used industry process to produce parts with hard outer cases and unhardened interior regions. During this process, eddy currents are induced into the surface layer of a steel part by passing alternating current through a closely coupled induction coil. The induced current heats the part from its surface to form an austenitic layer extending into the part from the surface, with the depth of the layer being a function of the frequency, power, and duration of the applied signal. Final hardening of the outer layer occurs when the power is shut off and the part is quenched from the outside to form a hardened martensitic structure.
Induction hardened parts often are designed to have a hardened layer within a desired depth. For example, a 25 mm diameter axle may be designed with a hardened layer from 4 to 5 mm thick. Should the layer be too thin, the axle would wear too quickly or have insufficient strength; should the layer be too thick, the axle would be too brittle.
Control of the hardening process has been an elusive goal of the industry for many years. Existing induction hardening equipment is typically operated with open-loop process controllers wherein an operator manually selects power and time. Production users of this equipment monitor the process by destructively sectioning parts and inspecting the results; i.e., a finished part is cut apart and the case depth is measured on the cross section. Process development for new parts is accomplished by time-consuming and expensive trial-and-error; for a given coil and part design, heating and quenching parameters are varied until destructive analysis reveals that the desired hardness distribution is being produced. These parameters are then utilized in the production run and the hardened parts are sampled and analyzed at regular intervals. If the tested part is bad, the production run from the previously tested good part is sampled to determined where the process failed. In addition, the production equipment may be taken out of service until subsequent parts test satisfactory. Since each test can take a minimum of several minutes by a trained technician, this process is quite inefficient in production operation.
An active feedback of information from the part and control of the heating process would greatly improve the efficiency of induction hardening systems. J. D. Verhoeven et al., Induction Case Hardening of Steel, Heat Treating, Vol. 4, No. 3, June 1986, pp. 253-262, used a transformerless system and measured the current and voltage at the heating coil, and the temperature of the part. They determined that case depth is proportional to the integral of the power density of the part over the heating time, and that power density is proportional to the square of the applied current. They postulated that a microprocessor could be used to terminate the system power at a predetermined time based on applied current.
U.S. Pat. No. 4,816,633 of George Mucha et al. discloses a method of monitoring an induction heating cycle by digitizing an analog signal representative of the applied voltage and comparing the resulting signal with a range of preselected patterns. A pattern outside the preselected pattern range indicates a problem with the process.
Peter Hassell, Potential of Monitoring Induction Heating in Real Time, Industrial Heating, January 1984, pp. 42-45, also discusses the desirability of determining comparative signatures of voltages and currents for the induction heating process, storing these signatures in a computer, and the potential of using the signatures to control the heating process. However, this reference does not discuss a specific method for monitoring signals or controlling the process in real time.
Until the present invention, there have been two significant limitations on the implementation of feedback control as proposed by Hassell and Verhoeven: it is very difficult to determine the power at the part, as the production machine includes a transformer, and the heating coil surrounds the part; and once the austentic phase begins, the increase in layer thickness occurs very quickly, so the system must respond very quickly. There is no showing in the prior art of a methodology by which the induction hardening process can be controlled in real time. Characteristic signals were known to exist, but there had been no showing of what they meant, how to track them in real time, or how to control the process based on them.